Implement initial HFS+ decmpfs (transparent compression) support.

This commit adds implementations for decmpfs compression methods 3 and
4, both of which are present in volumes created by Mac OS X 10.6 and
later.

Compression method 3 stores the file data inside the decmpfs attribute
in the attributes file and is only used for very small files. The file
data may be stored uncompressed or with zlib compression applied.

Compression method 4 stores the file data inside the file's resource
fork, as the resource with type 'cmpf'. Compression is applied on a
block-by-block basis, where each individual block can be stored
compressed (zlib) or uncompressed. Each compressed block can be
decompressed independently of the other blocks.

Volumes created by Mac OS X 10.10 are known to contain files compressed
with the so far undocumented compression methods 5, 7 and 8. Entries
compressed with these compression methods are unsupported.